Make default extension for page file configurable

These changes allow a user to change page extension from `page` to some other string (like `md`) in the configuration file. 

I think I've found all the places where `page` was hardcoded and adjusted functions accordingly, but there are still at least two problems I could use help with:

- It's hard to see an easy way to get rid of the hard-coded `page` extensions in the `Feed` module, so I've left that one in place even though it means links in the feed likely won't work.
- For some reason the `createIfMissing` function in `Initialize` doesn't seem to create the appropriate files with the newly configured extension, perhaps because it's using the default `Config` somewhere in the chain instead of reading config from file?

I've tested locally and the other functions that I changed still seem to behave as expected.